This invention relates to drive apparatus for selectively imparting rotary motion to two spaced apart rotary members and, more particularly, to bi-directional web driving apparatus which, in one embodiment, can be used as a tape transport.
In bi-directional driving apparatus, such as in web drive mechanisms, tape transport systems, and the like, it is desirable to provide bi-directional drive capability to the web or tape drive members. In some applications, it is often desirable to switch rapidly and efficiently from one driving direction to the opposite driving direction. For example, in a tape transport system which can be used in a tape recording/playback apparatus, it often is necessary for an operator to move tape in either fast forward or fast reverse direction. Such tape movement may be initiated immediately following a record or playback operation. Hence, this movement of tape is the well-known fast-forward or rewind modes wherein tape is selectively moved at a much faster rate than during the record or playback modes.
In some tape transport systems, a relatively low speed motor is utilized for the normal forward movement of tape, as during a record or playback mode. A fast-forward motor having a relatively higher speed is provided for the fast forward movement of tape and a rewind motor, also having a relatively higher speed, is provided for the rewind movement of tape. The selective movement of tape thus requires the selective energization of each of these motors. Consequently, it has been found that the inherent electrical and mechanical time delays result in a noticeable delay from the time that a motor initially is energized to the time that the corresponding tape movement is brought up to proper speed. This is particularly objectionable during a recording or playback operation. Also, the requirement of three motors adds to the bulk and cost of the tape transport system.
In a partial solution to this problem of time delays, some tape transport systems are provided with the low speed motor in constant operation. A selective transmission coupling is used to selectively transfer the rotary motion developed by this motor to various components of the system, such as the capstan/pressure-roll combination, the take-up reel, and the like. While this has minimized the delay attending a record or playback operation, the problem of separate high speed motors remains.
In many tape transport systems now in commercial use, the separate high speed motors have been replaced by a single bi-directional high speed motor having a selective transmission so as to couple the motor either to the supply or take-up reel. Heretofore, two solenoids have been used to control this selective transmission. When this type of tape transport system is provided, in, for example, a dictation machine, an additional solenoid must be provided to selectively position the record/playback head in contact with the tape, and sometimes still further solenoids are used for braking functions. Unfortunately, the use of many solenoids adds to the power requirements of the machine, increases the bulk and cost of the overall system, requires more complex control and support mechanisms, and, more significantly, is a source of distracting noise caused by the energization and de-energization of the various solenoids.
In an effort to reduce the number of solenoids in such machines, it has been proposed to control a solenoid so as to exhibit three conditions, or states, with the concurrent control of the tape transport system for each condition. This requires that the solenoid armature have a fully extended displacement, a fully retracted displacement and an intermediate displacement. Since, in general, a low-cost solenoid inherently is provided with two conditions wherein its armature either is fully extended or fully retracted, the requirement of an additional condition in such a solenoid is attended by relatively complex and costly control electronics. Furthermore, there is the additional problem that the intermediate position of the solenoid armature may, over a period of time, vary. Consequently, the mechanical function which is to be controlled by this intermediate armature position may be erroneously performed.
In addition to the aforedescribed undesirable aspects of prior tape transport devices such prior tape transport devices also have the disadvantage that the rapid actuation of mechanical transmissions invariably creates transient noise which is undesirable in a recording machine. Additionally, substantial power is consumed by these solenoid actuating means.